Electronic oscillator synchronizing circuit



Nov. 10, 1953 A. G. EMSLIE 2,659,009 ELECTRONIC OSCILLATOR SYNCHRONIZING CIRCUIT I Filed May 17, 1945 FIGJ POSITIVE BAS L HF 56 DAMPING PULSE INPUT INVENTOR.

' ALFRED G- EMSLIE BY ATTORNEY Patented Nov. 10, 1953 2,659,009 ELECTRONIC OSCILLATOR SYNCHRONIZING vCIRC UIT Alfred G. Emslie, Boston, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application May 17, 1945, Serial No. 594,260 9 Claims. (01. 250-36) The present invention relates to synchronization of oscillators, and it relates more particularly to oscillators that are synchronized by means of short-duration pulses of electrical energy.

One application of pulse-locked oscillators is in a coherent pulse-echo, radio object-location system. In a system of this general type, exploratory pulses consisting of short-duration high carrier-frequency pulses of radiant energy are transmitted, and simultaneously with said transmission, continuous-wave or interrupted continuous- Wave reference oscillations are initiated at the receiver. These reference oscillations must have a fixed-phase with respect to the transmitted exploratory pulses and hence are synchronized thereto. Echo pulses returning from a reflecting object are combined with the reference oscillations and a video pulse is obtained from said combination.

If the reflecting object is stationary, the video pulses representing said object will have a constant amplitude, whereas if the reflecting object is moving, the video pulses will have varying amplitudes. For a moving-object, the amplitude variation of the video pulses is periodic, the frequency of this periodic variation providing an indication of the radial velocity of the object relative to the system. A coherent pulse-echo system of this general character is more fully described in copending application of Robert H. Dicke, Serial No. 590,052, filed April 24, 1945, and issued on December 26, 1950, as Patent No. 2,535,274.

If the returning echo pulses are to be combined directly with the reference oscillations, said oscillations may be provided by a suitable radio-frequency oscillator. In some coherent pulse-echo systems, however, the carrier-frequency of the returning echo-pulses is reduced to a suitable intermediate-frequency, and in this instance the reference oscillations are generated by an intermediate-frequency oscillator. In either case, itis necessary to synchronize or look the reference oscillations with the transmitted exploratory pulses so that the reference oscillations will always have a fixed-phase with respect to said transmitted pulses.

It is therefore an object of the present invention to provide an improved pulse-injection method and system for synchronizing an oscillator readily and accurately to the exploratory pulses in such a manner that distortion is mini mized.

It is another object of the present invention to provide means for damping the reference oscillations immediately prior to injection of the syn chronizing pulses.

The above and other objects and advantages will appear more fully from the following description, considered with the accompanying drawings, wherein:

Fig. 1 is a schematic diagram of one embodiment of the present invention utilizing a diode;

Fig. 2 is a schematic diagram of a second embodiment of the present invention utilizing a cathode follower; and

Fig. 3 is a schematic diagram of a third embodiment of the present invention.

Referring now to Fig. 1 there is shown a pentode vacuum tube NJ for the generation of continuous wave oscillation. One side of grid coupling capacitance I3 is connected to control grid [8 of vacuum tube ill, the other side of capacitance It being connected to junction point Is. Junction point I9 is thence connected to ground through an oscillatory circuit comprising inductance I2 and parallel connected capacitance ll. Cathode K is connected to ground through an intermediate point on inductance l2. Grid circuit resistance It is connected between the terminals of grid [8 and cathode K. Anode 28 is connected to a positive source of potential through inductance l5; screen grid 3! is connected to the positive source of potential through resistance It and to ground through screen grid capacitance ll.

Anode 20 of amplitude-control diode vacuum tube 2| is connected to both junction point I9 and, through input coupling condenser 26, to input terminal 21. may be connected to ground through capacitance 25, and in addition may be provided with a positive bias voltage by connecting said cathode through resistance 24 to a suitable source of bias potential 25. Vacuum tube 2| then remains nonconducting until the voltage on anode 2B of said tube exceeds cathode bias potential 25.

The amount of positive bias voltage on cathode 22 of vacuum tube 2| establishes the magnitude of the oscillating voltage in the tank circuit comprising capacitance II and inductance i2. If the oscillator, of which said tank circuit is a component, is to provide reference oscillations for use with a coherent pulse-echo system of the character described, said cathode bias voltage can be set so that the oscillating voltage level will produce the desired signal magnitude at the mixer (not shown).

If the magnitude of the positive going portion of the tank circuit oscillations exceed the positive bias voltage on cathode 22 of vacuum tube Cathode 22 of vacuum tube 25' ,plitude-control tube :21. negative-going damping .pulse, iand prior to application of the synchronizing pulse, theoscilla- 2|, said vacuum tube becomes conducting, and its impedance becomes relatively small. Thus a low impedance is placed across the tank circuit and the oscillations therein are damped. Shortly after the magnitude of the thus damped oscillations becomes less than said cathode bias voltage, vacuum tube :21 again becomesnon-conducting, and the damping impedance is .removed. Vacuum tub 2|, therefore, provides a means whereby the tank circuit oscillations are maintained at a substantially .constantpredetermined amplitude.

Two pulses may be applied to input terminal 21, a negative-going dampingpulseanidzasynchronizing pulse, the damping pulse:bingapplied just prior to the synchronizing pulse. "in order to most easily lock the oscillator the carrier frequency of the synchronizing pulse should be substantially equal to the frequency -ofithe oscillator. The damping pulse feeds through input coupling capacitancezfi and grid coupling capacitance l3 to decrease the voltage on' control grid 18 of vacuum tube Thexefiect of this negative-going pulse is to momentarily damp the amplitude of the tank :circuit oscillations to a magnitude appreciably smaller than that .determined by the bias voltage on cathode 22:01am- Upon :removal :of .zthe

tions will quickly tend to rise to atheinormalmag- -,nitude as determined :by the-aforesaid .bias' voltage.

:Shortly after the damping pulseris applied to :input terminal 21 and before thertank. circuit toscillations have attained :theirwnormal amplitude the synchronizing ..pulse is applied to input Jae-rminal 21 through input coupling capacitance .26

and grid :couplingcapacitance |.3 rtoicontrolzgrid l8 .of vacuum tube 10. The synchronizing :pulse may, in a coherent-pulse iechosystem as-herctoforedescribed, be :a portion ofrthe transmitted pulse or a pulsesynchronizedathereto. .If the reference oscillations have :an intermediate :f-requency, the "carrier frequency. of .:the zporition irof the transmitted pulse 1 used to synchronize ithe "reference oscillator should. be reduced to ither-in- 'termediat frequency.

The striking :of :control :grid 18 by ithersynchronizing pulse reinforces the oscillations ofzaihe tank circuit in such-a .manner that :the oscillatoryioutpu't from ianodel iszof vacuum tube ;|:='is

securely locked .to the input synchronizing :pulse. The oscillator output :may be taken irom anode :28 .of vacuum tube 10, through.output-capaci tance 29 to output terminal 30. The damping :of the tank circuit oscillations prior-to injection 'of 'the synchronizing pulse insures that said 'pulse is of large magnitude-as compared 'to the -oscillations and can thereby control the phase ef-said,

oscillations.

A second mode of operation :is ome u'tiiizing a single "pulse, wherein the synchronizing pulse may accomplish both the damping and synchronizing functions. In such aninstance a synchronizing pulse, having an amplitude which exceeds the bias on cathode :22 of vacuum-tube 2 is applied to input terminal 21. Vacuum tubeIZlltthus conducts and momentarily :places. a low zimpe'dance acros the tank circuitwcomprising :capacitance H and inductance 12. The Qsofctheztank circuit is thus reduced and the :phaserof. the-{tank .circuit oscillations :can be readily. controlled by the synchronizing :pulse.

:Referring now to Fig. 2,1:a :cathodefollower replaces the diode 2| of Fig. 1 to provide damping means. The oscillator circuit including vacuum tube 40 and associated circuit elements is the same as that shown in Fig. 1. Vacuum tube 4|, however, is a cathode follower having anode 42 connected to a suitable source of positive po- "-tential' 43 and cathode 44 .connected to junction point '45. It will be noted that junction point 45 is similar to junction point H) of Fig. 1.

Control grid 46 of vacuum tube 4| is connected to .asourcc of negative bias potential 41 which ib'rase' vacuum'tiibe 4| beyond cutoff and establishes the amplitude of the tank circuit oscilla- .itions. .IWhen ithexnegative going portion of the oscillations drives cathode 44 of vacuum tube 4| suiiiciently negative, the potential difference between .c0ntr01,.grid 46 and cathode 44 is decreased to a value such that vacuum tube 4| will con- =duct. Conduction of this vacuum tube provides,

in essence, a low impedance across the tank circuit comprising inductance 48 and capacitance 49. The reason for this :is apparent when it is realized that the impedance flo.oking.from the tank circuit into the cathode .circuitxiszsubstan .tially so ineifect .a low impedanceis connected from .junction point 4.5.130 ground. .As is weli known in the ,art, gm .is defined. as the mutual. conduct- .ance of.a vacuum tube.

The eiiectof this .low impedance is .toudamp .the amplitude .ofoscillations in .the tank circuit.

Whenlthe magnitude of the negative going por- .tion..of these .oscillationsdecreases to .a -va-lue whereby the ,potentialdiiierence between control grid and cathode 4410f vacuum tube 4.] .makes said vacuum tube once again non-conducting, the normal amplitude of oscillations will. be resumed. .It will be seen therefore that .the effect of cathode followerk4| is vtomaintain the amplitudeof oscillations .in the tank circuit substantially constant, andsaid amplitude :maybe .0011- trolled'bybiassource4'l.

A positive-going damping pulse maybe applied .to "input terminal. and thence through resist- .ance .55 to control grid 4.6 of vacuum tube 4| to ing and synchronizing :circuit :shown in Fig. :1.

. Referring nowtoFig.. 3, vacuum 111138.169 .-'is a pentode, anode 6| of which may be -:connec.ted through anode inductance B2 to ,a ESQHIKCBiOf suit- .able positive potential zfi3. .Cathode- 64 of-vsaid vacuum tube may v:be connected through ,parallel connected cathode resistance-.65 and cathode capacitance .66 toeground. Y

A positive-going damping pulse mayabe applied .to .input terminal-'61 through coupling capacitance .68 .andgrid ileak rcsistancenfiS ito ground. The positive-going pulse developed across grid leak resistance 69 is thence applied to control grid 70 of vacuum tube 69. Appearing at anode 0 I of said vacuum tube is a negativegoing dampin pulse which is fed through output capacitance i! to the oscillator circuit in the same manner as was described in connection with the embodiment shown in Fig. 1. Similarly a synchronizing pulse is fed to input terminal 51 subsequent to said damping pulse.

The pentode has the advantage that during the time diode 12 is non-conducting, there is a very high impedance across the tank circuit comprising tznk circuit capacitance 13 and tank circuit inductance M which therefore isolates said tank circuit from all preceding circuits. This has the advantage in that it results in greater oscillator frequency-stability.

As described in connection with the embodiment of Fig. 1 a single pulse may be utilized to accomplish both the damping and synchronizing functions.

Although this method of oscillator synchronization is particularly applicable to coherent pulseecho object location systems, the method is perfectly general and may be applied to other oscillators. Furthermore, although Hartley type oscillators are shown in the drawings, other types of oscillators may be used in this invention with equal success. It is to be further understood that the synchronizing pulse need not be applied to the control grid of the oscillator tube but may be applied to other electrodes of said tube.

Having described the invention, what is claimed as new and is desired to be secured by Letters Patent is:

1. An oscillator synchronizin circuit, including a vacuum tube oscillator having an oscillation frequency determining tank circuit connected to the electrodes of said tube, normally inoperative damping means connected across said tank circuit for lowering the output of said oscillator, when it exceeds a predetermined amplitude, and means for applying pulses to an electrode of said tube to lower the amplitude and easily synchronize the oscillation of said oscillator with said pulses.

2. An oscillator synchronizing circuit, including a vacuum tube oscillator having an oscillation frequency determining tank circuit connected to the electrodes of said tube, damping means comprising a vacuum tube connected in shunt with said tank circuit and means for applying a predetermined cut-off bias voltage to said shunt vacuum tube, whereby said shunt vacuum tube serves to damp the amplitude of oscillation of said oscillator when said bias is overcome, and means for applying pulses to an electrode of said oscillator tube to lower the amplitude and easily synchronize the oscillation of said oscillator with said pulses.

3. The circuit of claim 2, wherein said shunt vacuum tube comprises a diode.

4. The circuit of claim 3, wherein said pulses comprise a series of damping pulses each of which is immediately followed by a synchronizing pulse having a carrier frequency substantially equal to said oscillation frequency.

5. The circuit of claim 2, wherein said shunt vacuum tube comprises a triode having its cathode connected to said means for applying pulses to said oscillator tube and its anode connected to a source of positive potential.

6. The circuit of claim 5, wherein said pulses comprise a series of damping pulses each of which is immediately followed by a separate synchronizing pulse having a carrier frequency substantially equal to said oscillation frequency, said damping pulses being applied to the control electrode of said triode and the synchronizing pulses being applied directly to said means for applying pulses to said oscillator tube.

7. The circuit of claim 2, further including a pentode vacuum tube in shunt with said tank circuit and receptive of said pulses, for coupling said pulses to said means for applying pulses to said oscillator tube and for isolating said tank circuit when the first-named shunt vacuum tube is cut-off.

8. The circuit of claim 7, wherein the firstnamed shunt vacuum tube comprises a diode.

9. The circuit of claim 8, wherein said pulses comprise a series of damping pulses each of which is immediately followed in time by a separate synchronizing pulse having a carrier frequency substantially equal to said oscillation frequency.

ALFRED G. EMSLIE.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Feb. 2, 1937 

